A filter unit fitted to an internal combustion engine for filtering the lubrication oil generally comprises a filter housing and a filter element accommodated therein. The lubrication oil led into the housing from the engine is filtered by the filter element and returns to the inside of the engine.
Since the filter unit is filled with lubrication oil while in operation, the lubrication oil may flow out and scatter when the filter unit is removed from the engine for replacement.
In this context, Tokkai Hei 9-173716 published by the Japanese Patent Office in 1997, discloses to provide a drain plug at the bottom of the filter housing. When the filter unit is to be replaced, the drain plug is removed, thereby evacuating the filter unit, and then the empty filter unit is removed from the engine.